Semiconductor fabrication continues to strive to make individual electronic components smaller and smaller, resulting in ever denser integrated circuitry. One type of integrated circuitry comprises memory circuitry where information is stored in the form of binary data. The circuitry can be fabricated such that the data is volatile or non-volatile. Volatile storing memory devices result in loss of data when power is interrupted. Non-volatile memory circuitry retains the stored data even when power is interrupted.
This invention was principally motivated in making improvements to the design and operation of memory circuitry disclosed in the Kozicki et al. U.S. Pat. Nos. 5,761,115; 5,896,312; 5,914,893; and 6,084,796, which ultimately resulted from U.S. patent application Ser. No. 08/652,706, filed on May 30, 1996, disclosing what is referred to as a programmable metallization cell. Such a cell includes opposing electrodes having an insulating dielectric material received therebetween. Received within the dielectric material is a fast ion conductor material. The resistance of such material can be changed between highly insulative and highly conductive states. In its normal high resistive state, to perform a write operation, a voltage potential is applied to a certain one of the electrodes, with the other of the electrode being held at zero voltage or ground. The electrode having the voltage applied thereto functions as an anode, while the electrode held at zero or ground functions as a cathode. The nature of the fast ion conductor material is such that it undergoes a structural change at a certain applied voltage. With such voltage applied, a single conductive dendrite or filament extends between the electrodes, effectively interconnecting the top and bottom electrodes to electrically short them together.
Once this occurs, dendrite growth stops, and is retained when the voltage potentials are removed. Such can effectively result in the resistance of the mass of fast ion conductor material between electrodes dropping by a factor of 1,000. Such material can be returned to its highly resistive state by reversing the voltage potential between the anode and cathode, whereby the filament disappears. Again, the highly resistive state is maintained once the reverse voltage potentials are removed. Accordingly, such a device can, for example, function as a programmable memory cell of memory circuitry.
The preferred resistance variable material received between the electrodes typically and preferably comprises a chalcogenide material having metal ions diffused therein. A specific example is germanium selenide having silver ions diffused therein. The present method of providing the silver ions within the germanium selenide material is to initially chemical vapor deposit the germanium selenide glass without any silver being received therein. A thin layer of silver is thereafter deposited upon the glass, for example by sputtering, physical vapor deposition or other technique. An exemplary thickness is 200 Angstroms or less. The layer of silver is irradiated, preferably with electromagnetic energy at a wavelength less than 500 nanometers. The thin nature of the deposited silver enables such energy to pass through the silver to the silver/glass interface effective to break a chalcogenide bond of the chalcogenide material. This may form Ag2Se, which effectively dopes the glass with silver.
Saturation of silver in germanium selenide is apparently at a maximum of about 34 atomic percent or less depending on the germanium selenide stoichiometry. Yet, preferred existing technology for cell fabrication constitutes a concentration which is less than the maximum; in the case of 34 atomic percent maximum, an example concentration would be about 27 atomic percent.
After the chalcogenide material is provided with silver to a desired concentration, the top electrode material (typically silver) is next deposited. But, as the silver doping/diffusion into the chalcogenide material approaches the maximum or saturation, some Ag2Se was discovered to form at the surface and remain there as opposed to diffusing into the glass. Further, the surface Ag2Se was typically in the form of semicircular nodules or bumps anywhere from 50 Angstroms to 20 microns across. Unfortunately when the typical silver electrode material is subsequently deposited, such tends to mound on top of these previous bumps. This can create voids to the doped germanium glass through the top electrode material, whereby the silver doped germanium selenide glass is partially exposed. Unfortunately, some of the photodeveloper solutions typically used for patterning the top electrode (i.e. tetramethyl ammonium hydroxide) will etch the glass that is exposed.
It would be desirable to overcome or at least reduce this problem. While the invention was principally motivated in overcoming this problem, it is in no way so limited. The artisan will appreciate applicability of the invention in other aspects unrelated to the problem, with the invention only being limited by the accompanying claims as literally worded and as appropriately interpreted in accordance with the doctrine of equivalents.